1. Field of the Invention
This invention relates to a heat conduction pipe and a heat exchanger assembled with said heat conduction pipes for performing thermal exchange between cooling water, cooling air, cooling medium for air conditioner, or other cooling mediums and EGR gas (Exhaust Gas Recirculation) gas or combustion exhaust gas including soot in a mulitube heat exchanger such as an EGR cooling mechanism and so on.
2. Description of Related Art
Conventionally, EGR systems in which a part of exhaust gas is taken out of an exhaust gas system and returned to an intake system of the engine to be added to the mixture gas and the intake air, have been used in engines for automobiles along with gasoline engines and diesel engines. With the EGR system, particularly with the cooled EGR system for diesel engines of high EGR rate, a cooling apparatus for cooling the EGR gas at a high temperature with cooling water, cooling air, refrigerants, or other cooling mediums is provided to reduce nitrogen oxide gas (NOx). in the exhaust gas, to prevent the mileage from becoming inferior, and to prevent functions and durability of the EGR valve from deteriorated due to excessively increased temperature.
As shown in FIG. 3, there is an EGR gas cooling apparatus that performs thermal exchange between the EGR gas and the cooling medium via heat conduction pipes by disposing a plurality of the heat conduction pipes having smaller diameters through the interior of which the EGR gas can communicate and by communicating flowing the cooling mediums such as cooling water, cooling air, refrigerants, etc., through the exterior of the heat conduction pipes.
As the heat conduction pipes like above, arts as described in Japanese Patent Application Publication Nos. JA-11-108,578, JA-2001-227,413, and JA-2002-28,775, and European Patent No. 1265,046 A2 have been known. With these conventionally known heat conduction pipes, the inner circumferential surface through which liquid can communicate is smooth, so that soot contained in the communicating exhaust gas may deposit thereon. Where the soot deposits on the inner surface of the heat conduction, the soot creates a heat insulation effect to reduce a thermal conduction efficiency, so that it is undesirable as diminishing functions in terms of the heat conduction pipe. Therefore, as a method to remove the soot from the inner surface of the heat conduction pipe, conventionally, adapted is to wipe the deposited soot with a member in a blush shape after use of the heat conduction pipe for a predetermined period, or to heat the heat conduction pipe to burn the soot by stopping a cooling operation therein.
However, with the method for wiping the soot off using the blush or the other, or for heating the heat conduction pipe to bum the soot by stopping a cooling operation therein, the workability of the heat conductor pipe may be significantly reduced, not only because many processing steps are needed but also because the cooling operation in the heat conductor pipe has to be stopped. With the aim of prevention of such defects so as to prevent the soot from depositing on the inner surface of the heat conductor pipe, practiced is to form a low energy coating such as fluoric resin on the inner surface of the heat conduction pipe. However, with the method for forming the low energy coating on the inner surface of the heat conduction pipe, the thermal conduction rate and the thermal conductivity of the low energy coating such as fluoric resin are low compared to metal, so that the thermal conduction efficiency of the heat conduction pipe as of essentially a heat exchanger may be reduced.